This invention relates generally to nuclear medicine imaging systems and, more particularly, to methods and apparatus for providing coincidence imaging in such systems.
Positrons are positively charged electrons that are emitted by radionuclides that have been prepared using a cyclotron or other device. These positions are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected into a patient and become involved in such processes as blood flow, glucose metabolism, and protein synthesis.
Positrons are emitted as the radionuclides decay. The positrons normally travel a very short distance before they encounter an electron, and when this occurs, the positron and electron are annihilated and converted into a pair of photons, or gamma rays. This annihilation “event” is typically characterized by two features that are pertinent to positron emission tomography (PET) scanners, namely, each gamma ray has an energy of 511 keV and the pair of gamma rays are directed in substantially opposite directions. An image is created by determining the number of such annihilation events at each location within the field of view.
A PET scanner may include two or more solid-state or scintillation detectors to detect individual photons. Some known scanners include a plurality of detectors that define a ring around a volume of interest. Timing the detection of these events is used to identify the pairs of photons from a single annihilation. To facilitate effective operation, detection events should be able to be timed to ten nanoseconds or less. Each scintillation detector includes a scintillator that converts the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits are coupled to the detectors and record only those photons that are detected simultaneously by two detectors located on opposite sides of some part of the patient. The number of such simultaneous events indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. Within a few minutes hundreds of millions of events may be recorded to indicate the number of annihilations along lines joining pairs of detectors in the ring. These numbers of events are employed to reconstruct an image using, for example, computed tomography techniques.
At least some known coincidence imaging systems use analog electronic circuits to generate a timing signal resulting from the sharp rise in output voltage in the detection device when the detection device detects an event. However, such circuits may have a limited accuracy when the event rate in the detector is very high. Typical trigger circuits often fail to detect events occurring immediately after a preceding event, and when they do detect such events, they detect an event time later than the actual time.